1. Field of the Invention
The present invention relates to an image forming apparatus and method for forming a color image on a recording medium using recording agents of a plurality of colors.
2. Related Background Art
A laser beam printer (LBP) is a well known example of present-day conventional image forming apparatuses that use color image data for printing. When one type of laser beam printer is used to print a color image, for each of the colors that are to be printed a main scan means employs a laser beam, which uses a rotary polyhedron mirror, to expose a photosensitive member and to form on the member a latent image composed of a series of scan lines for a specific individual color. Then, again for each of the colors, the latent image is developed by using either a magenta (M), a cyan (C), a yellow (Y) or a black (BK) color developer to form an individually colored image that, subsequently, is transferred to a recording sheet, securely held on a transfer drum, on which all the individually colored images are superimposed, one on the other. Another type of color image apparatus temporarily transfers images for individual colors from a photosensitive member to an intermediate transfer member, superimposing the images thereon, and collectively transfers the color images from the intermediate transfer member to a recording sheet.
For these apparatuses, the photosensitive member and the transfer drum, and the intermediate member, if used, are moved at a constant speed in the direction (the sub-scan direction) perpendicular to the main scan direction, while an optical system scans an original document at the same speed.
When, at each revolution, the photosensitive drum, the transfer drum or the intermediate drum generates a sub-scan start reference signal, in synchronization with this signal, the optical system initiates the scanning of an original document, and a CCD (charge-coupled device) begins to read line data. Similarly, in synchronization with the sub-scan start reference signal, data for each color are recorded on a recording sheet held on the transfer drum, or on the intermediate member. In this manner, the timing for the data reading and for the data writing are synchronized.
For the additional image forming apparatus that transfers individually colored images to a photosensitive drum, superimposing them thereon, and collectively transfers the resultant image to a recording sheet, to prevent deterioration of the quality of a color image, which occurs when the positions are shifted at which images are read for the individual colors, a method that controls the position at which an image for each color is read is important.
According to a conventional reading position control method, rotation precision is increased for a polygon motor that generates, as a reference for main scan sync control, a main scan start reference signal (hereinafter referred to as a BD (beam detect) signal), and a drum motor that generates, as a reference for sub-scan sync control, a photosensitive drum rotation position signal (hereinafter referred to as a sub-scan start reference signal (an ITOP (image top) signal)). In addition, the drum motor and an optical motor that drives an original reader for scanning an original document are controlled at the same speed.
FIG. 12 is a diagram showing an example arrangement of a system for driving a polygon motor and a drum motor in a conventional image forming apparatus.
In FIG. 12, a photosensitive drum 1301 is rotated via a drive belt 1308 by a photosensitive drum drive motor 1307. A polygon motor 1302 rotates a polygon mirror 1303 that, for each line of a document, scans, the face of the photosensitive drum 1301 with a beam emitted by a laser 1304 and transmitted via a lens 1305.
A PLL (phase-locked loop) circuit 1310 drives the polygon motor 1302 at a constant speed based on a reference clock received from an oscillator 1311, and a PLL circuit 1309 drives the photosensitive drum drive motor 1307 at a constant speed based on a reference clock received from an oscillator 1312. The rotations of the polygon motor 1302 and of the photosensitive drum drive motor 1301 are matched in consonance with the precision of the frequencies generated by the oscillators 1311 and 1312.
With this arrangement, since the photosensitive drum 1301 and the polygon motor rotate at a controlled, constant speed, the ITOP signal, which is synchronized with the rotation of the photosensitive drum 1301, and the BD signal, which is synchronized with the rotation of the polygon motor 1302, are produced at predetermined frequencies.
Therefore, the conventional image forming apparatus employs the ITOP signal to control the original document scan start timing for the optical system, and the line data reading start timing and the image writing timing for a three-line sensor (hereinafter referred to as CCD), so that the reading positions for individual colors correspond. It should be noted that the CCD includes a light accumulation unit for acquiring, as data, light reflected from an original document that is being scanned, and a transfer unit for outputting, as writing data, the data that are acquired by the light accumulation unit.
An explanation will now be given, while referring to FIGS. 13 to 16A and 16B, for an example original document reading method employed by the conventional image forming apparatus.
FIG. 13 is a timing chart for explaining the line data fetching process performed by the CCD of the conventional image forming apparatus.
In FIG. 13, a CCD light accumulation/transfer control signal (hereinafter referred to as a control signal) is synchronized with a BD signal. Light is accumulated (line data are fetched) for a predetermined time period during an interval in which the control signal is at level H, and then, the accumulated line data are transmitted to the transfer unit during an interval in which the control signal is at level L. During the next light accumulation period, line data are output as writing data by the transfer unit at a delay, relative to the line accumulation process, of one line (one BD cycle).
Specifically, the control signal is synchronized with the BD cycle to repeat the light accumulation process (a level H interval) and the data transmission process (a level L interval).
First, at interval (1), during which the control signal is at level H, the CCD accumulates data for an original document that is being currently scanned by the optical system, i.e., xe2x80x9creading data 1xe2x80x9d in FIG. 13. Then, at interval (2), during which the control signal is at level L, the CCD transfers the data acquired at interval (1) to the transfer unit.
At interval (3), during which the control signal goes to H again, data are accumulated for the original document that is currently being scanned by the optical system, i.e., xe2x80x9creading data 2xe2x80x9d in FIG. 13, and the xe2x80x9creading data 1xe2x80x9d, which were transmitted to the transfer unit, are output as xe2x80x9cwriting data 1xe2x80x9d. The same process is repeated at interval (4) and the following intervals, and in synchronization with the BD signal, each line of the original document is read and the writing data are output.
FIG. 14 is a timing chart for explaining the original document reading timing for the conventional image forming apparatus, in which is shown a BD signal, and ITOP signals, the read data and the write data for a first and a second color.
In FIG. 14, the BD signal, which is a main scan start reference signal, is generated at cycle T in synchronization with the rotation of a polygon motor. The ITOP signal, which is a drum rotation position signal, is generated at an arbitrary time during a BD signal cycle.
When the ITOP signal is changed from level H to level L, the optical system begins scanning, and the CCD initiates the fetching of line data, synchronized with the generation of the next BD signal. The scanning speed of the optical system and the rotation speed of the photosensitive drum are indicated by xe2x80x9cVxe2x80x9d.
Now, the reading/writing operation for the first color will be described.
When time T1 has elapsed, following the input of the ITOP signal for the first color, a main scan start reference signal (BD signal) is generated and the reading of data for the first line is begun. At this time, the scanning position of the optical system relative to the original document is shown in FIG. 15, which will be described later, as position xe2x80x9cAxe2x80x9d, i.e., position xe2x80x9cVxc3x97T1xe2x80x9d, calculated from the leading edge of the original document.
The data read for the first line are output, as first line writing data, by the CCD following a one line delay (i.e., when a time xe2x80x9cT1+Txe2x80x9d has elapsed following the entry of the ITOP signal for the first color). At this time, the beam is projected onto the photosensitive drum at position xe2x80x9cVxc3x97T1+Vxc3x97T (a one line delay relative to a CCD read)xe2x80x9d, separated from the ITOP signal generation position. The first line writing data are recorded on the drum beginning at the first color writing start position xe2x80x9cAxe2x80x2xe2x80x9d in FIG. 15, which will be described later.
The reading/writing process for the second color will now be described.
When time T2 has elapsed following the input of the ITOP signal for the second color, a main scan start reference signal (BD signal) is generated, and the reading of data for the first line is begun. At this time, the scanning position of the optical system on the original is position xe2x80x9cBxe2x80x9d in FIG. 15, i.e., position xe2x80x9cVxc3x97T2xe2x80x9d calculated from the leading edge of the original. The data read for the first line are output as first line writing data by the CCD following a one line delay (i.e., after the elapse of time xe2x80x9cT2+Txe2x80x9d following the entry of the ITOP signal for the second color). At this time, the beam is projected onto the photosensitive drum at position xe2x80x9cVxc3x97T2+Vxc3x97T (a one line delay relative to the CCD reading)xe2x80x9d, separated from the ITOP signal generation position. The first line writing data are recorded on the drum beginning at the second color writing start position xe2x80x9cB1xe2x80x9d in FIG. 15, which will be described later.
FIG. 15 is a specific diagram showing image reading positions and image writing positions for the conventional image forming apparatus.
As shown in FIG. 15, for the first color, data that are read beginning at point xe2x80x9cAxe2x80x9d on the original document are written beginning at point xe2x80x9cAxe2x80x2xe2x80x9d on the drum, and for the second color, data that are read beginning at point xe2x80x9cBxe2x80x9d of the original document are written beginning at point xe2x80x9cB1xe2x80x9d on the drum.
Assume that the left upper end of the original document is defined as an original document TOP, and position xe2x80x9cVxc3x97Txe2x80x9d, separated from the ITOP signal generation position on the photosensitive drum, is defined as a sheet TOP. Then, the points xe2x80x9cAxe2x80x9d and xe2x80x9cAxe2x80x2xe2x80x9d and the points xe2x80x9cBxe2x80x9d and xe2x80x9cBxe2x80x2xe2x80x9d are identical as viewed from the original document TOP and the sheet TOP. Further, since in the following process the rotation speed of the photosensitive drum is the same as the scanning speed of the optical system, the original document that is read in the same manner is recorded on the photosensitive drum at a location that corresponds to the reading position.
It should be noted that the reading start time difference xe2x80x9cT1xe2x88x92T2xe2x80x9d, between the first and the second colors, is only a shift of the write start position from the sheet TOP, i.e., only a difference corresponding to the margin, and a shift in the position of the complete image does not occur.
However, with the reading/writing technique of the conventional image forming apparatus, the MTF (modulation transfer function) for reading data for the individual colors is varied when the reading position on the original document differs for the first and the second colors (a detailed explanation of this will be given later while referring to FIGS. 16A and 16B).
Since the MTF for each color is changed, a defect occurs. For example, the reproduction of fine lines varies for each color, or the determination of edges, such as those of characters, is changed depending on the color.
FIGS. 16A and 16B are diagrams showing the relationship between the line reading position of the conventional image forming apparatus and data sampling. In FIG. 16A, the relationship between the original document reading timing and the original document reading position is shown, and in FIG. 16B, the relationship between the original document reading position and density data sampling values of the original document is shown.
In FIG. 16A, each pixel of the original document is divided into squares, and in each square the density data for a pixel is entered. Since actually, the color data to be read differs for the first and the second colors, basically, the density data for each pixel is different. However, for the sake of convenience, in this embodiment the pixel data for the first and the second colors are the same.
An explanation will now be given for the reading of the data for the first color.
As shown in FIG. 16A, the reading start timing for the first color begins after time T1 has elapsed following the generation of the ITOP signal. Actually, the reading of the original document begins at the point reached by a reader following the elapse of a time xe2x80x9cVxc3x97T1xe2x80x9d, in which xe2x80x9cVxe2x80x9d is the speed of movement of the reader, i.e., the line for pixel C. At this time, the main scan start reference signal (BD signal), for reading the first color, is generated as shown in FIG. 16A, each line of the original is read, and the obtained line data are transmitted as print data to a printer.
An explanation will now be given for data read for the second color.
As shown in FIG. 16A, the reading start timing for the second color begins after time T2 has elapsed following the generation of the ITOP signal. Actually, the reading of the original document begins at the point reached by the reader following the elapse of a time xe2x80x9cVxc3x97T2xe2x80x9d, in which xe2x80x9cVxe2x80x9d is the speed of movement of the reader, i.e., a position 0.5 line before the line for pixel C. At this time, the main scan start reference signal (BD signal) for reading the second color is generated as shown in FIG. 16A, each line of the original document is read, and the obtained line data are transmitted as print data to a printer.
Relative to the ITOP signal, the main scan start reference signal (a BD signal) for the second color is generated with a phase differing from that of the main scan start reference signal (a BD signal) for the first color.
In FIG. 16B, (b)-1 corresponds to a sampling value for image data read for the first color, and (b)-2 corresponds to a sampling value for image data read for the second color.
As shown in (b)-1, since for the first color the lines for pixels C and D in FIG. 16A have the same phase as the main scan start reference signal (a BD signal), data having the same density as that of the original document are read (sampled) at the same positions as are the data of the original document in FIG. 16A.
However, as shown in (b)-1, since for the second color the lines of pixels C and D in FIG. 16A are shifted 0.5 line away from the main scan start reference signal (a BD signal), one half of pixel C and one half of the preceding pixel are read as data for one pixel, the other half of pixel C and one half of pixel D are read as data for another pixel, and the other half of pixel D and one half of the succeeding pixel are read as data for one more pixel (sampling). The density data for the pixel obtained by reading (sampling) half of pixel C and half of the preceding pixel, or half of pixel D and half of the succeeding pixel, is xe2x80x9c50,xe2x80x9d while the density data for the pixel obtained by reading (sampling) the halves of pixels C and D is xe2x80x9c100xe2x80x9d.
It should be noted that the center density of the first color in (b)-1 is the same as that for the second color in (b)-2, and no color shifting occurs between the first and the second colors.
However, since the data xe2x80x9c0xe2x80x9d, xe2x80x9c100xe2x80x9d and xe2x80x9c100xe2x80x9d are obtained for the first color in (b)-1, while the data xe2x80x9c50xe2x80x9d, xe2x80x9c100xe2x80x9d and xe2x80x9c50xe2x80x9d are obtained for the second color in (b)-2, the resolution for the second color in (b)-2 is reduced.
This is because the phase at which the ITOP signal is generated relative to the BD signal differs for the first and the second colors and because the reading MTF is different. Therefore, a sharp image can be reproduced for the first color, while an ambiguous image is reproduced for the second color, so that the reproductivity varies depending on the color element.
In addition, the position determined for an edge may be changed, depending on the color.
For example, for the first color in (b)-1, the image data having a density 100 on the left side can be employed to determine an edge, while for the second color in (b)-2, the image data having a density 50 on the left side may also be employed to determine an edge.
As described above, for the conventional image forming apparatus, the image reproductivity differs depending on the color element, and the quality of an image may be degraded.
Also, as described above, a laser beam printer (LBP) is well known as a color image forming apparatus that uses color image data for printing. An LBP scans a photosensitive member by using a rotary polyhedron mirror (a polygon mirror) to reflect a laser beam; sequentially forms, on the photosensitive member, a latent image corresponding to one line of an image; and attaches one of several developers (toners), for example, magenta (M), cyan (C), yellow (Y) and black (BK), to a latent image composed of multiple lines (for one screen) to form a screen image for one color. To complete the printing of one color, a screen image for the color on the photosensitive member is transferred to a recording sheet that is securely held on a transfer drum. Then, the printing process is repeated for the remaining three colors, so that a color image expressed in four colors can be printed.
Another image forming apparatus is one whereby screen images for four individual colors formed on the photosensitive member are temporarily superimposed on an intermediate transfer member, following which the color images on the intermediate transfer member are collectively transferred to a recording sheet.
These apparatuses sequentially superimpose a plurality of screen images, while driving the components in the sub-scan direction. Specifically, the photosensitive member, the transfer member and the intermediate transfer member are driven at a constant speed in the direction (the sub-scan direction) perpendicular to the main scan direction. And the screen images are transferred from the photosensitive member to the transfer member, or the intermediate transfer member, whereon they are superimposed, in synchronization with a sub-scan start signal that is generated upon each rotation of the transfer member or the intermediate transfer member. In this manner, the position shifting of the screen images can be reduced.
As another color image forming technique, instead of latent color images being transferred individually, as they are formed on the photosensitive member, the images are superimposed, one on the other, so that a screen image composed of four colors is formed on the photosensitive member. Then, at the following stage, the superimposed color images on the photosensitive member are collectively transferred to the transfer member (sheet).
For the various color image forming techniques mentioned above, to provide as a final product a printed color image having a satisfactory quality, it is preferable, during the repetitive reading of an image performed for each color, that screen images for individual colors be read and superimposed with as little position shifting as possible.
In order to reduce the position shifting when screen images for individual colors are read, a specific method is conventionally employed. According to this method, rotational precision is increased for a scanner motor, which drives a rotary polyhedron mirror (a polygon mirror), for reflecting a laser beam to scan the photosensitive member, and which generates the main scan start signal (a BD signal), a reference for main scan sync control; and a drum motor, which drives a photosensitive drum and which generates a sub-scan start signal (an ITOP signal), a reference for sub-scan sync control. Further, the drum motor and an optical motor, which drives a line sensor in a document reader for reading an original color document, are controlled at the same speed.
FIG. 31 is a diagram showing the arrangement of a system in a conventional color image forming apparatus for driving a scanner motor and a drum motor.
In FIG. 31, a photosensitive drum 2105 is rotated by a drive motor 2115 via a drive belt 2116. A transfer drum 2108 rotates in contact with the photosensitive drum 2105, its rotary movement coupled with that of the photosensitive drum 2105. A transfer sheet 2109 is attached to the transfer drum 2108 at a predetermined position, and a toner latent image formed on the photosensitive drum 2105 is transferred to the transfer sheet 2109. An ITOP sensor 2110 shielded by a flag 2111 generates an ITOP signal that is synchronized with the rotational location of the transfer drum 2108, i.e., of the photosensitive drum 2105.
A PLL circuit 2114 rotates a scanner motor 2106 at a constant speed, based on a reference clock that is provided when a frequency division circuit 2113 divides a clock output by an oscillator 2112. The scanner motor 2106 drives a polygon mirror 2103, and for each line scans the face of the photosensitive drum 2105 by projecting a laser beam from a laser 2102 via a lens 2104.
A frequency division circuit 2119 divides a clock output by the oscillator 2112, and the obtained clock is employed as a reference clock for a PLL circuit 2118, which drives, at a constant speed, the drive motor 2115 that rotates the photosensitive drum 2105. In this manner, the rotation of the scanner motor 2106 is matched with the rotation of the drive motor 2115 in accordance with the precision provided by the oscillator 2112 and the frequency division circuits 2113 and 2119.
With this arrangement, since the photosensitive drum and the polygon mirror are rotated at predetermined constant speeds, the ITOP signal, which is synchronized with the rotation of the photosensitive drum, and the BD signal, which is synchronized with the rotation of the polygon mirror, are generated at predetermined cycles. The start timing of the optical motor, which employs the ITOP signal to drive the line sensor and scan the original document, the scan start timing of the line sensor, and the timing for recording screen images for individual colors to the photosensitive drum are controlled, so that the reading position and writing position for a screen image for each color correspond.
The operation of the conventional arrangement in FIG. 31 will now be described while referring to FIGS. 32 and 33.
FIG. 32 is a timing chart showing the relationship of an ITOP signal to the first to fourth colors, an ITOP signal for the first and the second colors, a BD signal, the start timing for an optical motor, a line sensor reading start signal, and a laser writing start signal.
In FIG. 32, the BD signal, which is a main scan start signal, is generated with a cycle T in synchronization with the rotation of a scanner motor. The ITOP signal, which indicates the rotational position of the photosensitive drum, is asynchronous with the BD signal, and is generated at an arbitrary timing during the BD cycle (i.e., in FIG. 32, the trailing end of the ITOP signal is generated in the middle of the BD cycle). The optical motor is started when a predetermined time Ts has elapsed following the changing of the ITOP signal from level H to level L. The reading of data by the line sensor is begun in synchronization with the n-th BD signal, calculated from the generation of the ITOP signal, and data for each line is read, in synchronization with the BD signal, in a number equivalent to a predetermined read line count. That is, when the BD cycle is defined as T, data reading is begun after time Nxc3x97T+(T/2) has elapsed.
The reading operation performed by the line sensor will now be described while referring to FIG. 33.
In FIG. 33, a light accumulation/transfer control signal for the line sensor is synchronized with the BD signal. Light is accumulated (data are fetched) for a predetermined time period (during a period in which the control signal is at level H), and the accumulated data are transmitted to the transfer unit during a predetermined time period (a period in which the control-signal is at level L). The data received by the transfer unit are transferred during the next light accumulation process, and are output as writing data with a delay of p lines (pxc3x97BD cycle) following the light accumulation process.
Specifically, in synchronization with the BD cycle, the light accumulation/transfer control signal of the line sensor repeats the accumulation of light and the transmission of data to the transfer unit. Interval (1) of the control signal is a light accumulation interval, and during this period, the line sensor accumulates data for the original document that is currently being scanned, i.e., xe2x80x9creading data 1xe2x80x9d in FIG. 33.
In interval (2) of the control signal, the data obtained in interval (1) are transmitted to the transfer unit. In interval (3), wherein the control signal goes to level H again, the data for the original document that is currently being scanned by the line sensor, i.e., xe2x80x9creading data 2xe2x80x9d, are accumulated, and the xe2x80x9creading data 1xe2x80x9d transmitted to the transfer unit are output as xe2x80x9cwriting data 1xe2x80x9d. The same process is performed for interval (4) and the following intervals, and data are read for each line, in synchronization with the BD signal, and output to a laser driver (not shown).
The timing for recording the data that are read will now be described while referring to FIG. 32.
An image process, such as tone correction, is performed for the image data read by the line sensor, and the resultant image data are transmitted as laser writing data to the laser driver (not shown). In synchronization with the n-th BD signal obtained following the generation of the ITOP signal, each line of the image data that corresponds to the BD cycle is recorded on the photosensitive drum. That is, the recording on the photosensitive drum is begun after time mxc3x97T+(T/2) has elapsed following the generation of the ITOP signal. Therefore, for the reading/writing operation for the first color, the data that were read after time nxc3x97T+(T/2) had elapsed following the generation of the ITOP signal are written on the photosensitive drum when time mxc3x97T+(T/2) has elapsed. The same process is performed for reading and writing data for the second and the following colors. However, since the ITOP signal and the BD signal are asynchronous with each other, the phases of the ITOP signal and the BD signals differ from those for the first color, and, for example, the phase for the second color is shifted 1.4 line, as shown in FIG. 32. In this case, as well as for the first color, the optical motor starts after real time Ts has elapsed following the input of the ITOP signal. The line sensor starts data reading when time nxc3x97T+(T/4) has elapsed following the input of the ITOP signal, and the laser writing starts when time mxc3x97T+(T/4) has elapsed.
For image forming performed by superimposing images for the first and the second colors, for the first color, an image that was read in synchronization with the BD signal after time nxc3x97T+(T/2) had elapsed is written in synchronization with the BD signal after time mxc3x97T+(T/2) has elapsed. For the second color, an image that was read in synchronization with the BD signal after time nxc3x97T+(T/2) had elapsed is written in synchronization with the BD signal after time mxc3x97T+(T/2) has elapsed. Since the scanning of the original document by the line sensor, and the rotation of the photosensitive drum are operated at the same speed Vs in real time relative to the generation of an ITOP signal, the original scanning position and the rotational position of the photosensitive drum for the first and the second colors are the same in real time following the generation of the ITOP signal. However, since the reading start position and the writing start position are determined in synchronization with the BD signal, the phases of the ITOP signal and the BD signal affect these positions, so that the reading start position and the writing start position for the second color are shifted a distance d from those for the second colors, as shown in FIG. 34. This shifting distance d (=Vsxc3x97T/4) is obtained by multiplying difference T/4, between time nxc3x97T+(T/2) and time nxc3x97T+(T/4), by the scanning speed of the line sensor=the rotational speed of the photosensitive drum=Vs.
With the first color employed as a reference, for the second color, data that were read at a forward position a distance d from the position of the first color are written at a forward position a distance d from the position for the first color. The data that are read are written at a corresponding position on the photosensitive drum.
The same process is performed when the difference in the phases of the ITOP signal and the BD signal for the third and following colors is xcex1xc3x97T (xcex1 less than 1).
As described above, in the conventional color image forming apparatus, the original scanning performed by the line sensor is started based on using the ITOP signal as a reference, and the data reading timing and the timing for writing to the photosensitive drum are synchronized with the BD signal based on the ITOP signal, so that the reading position and the writing position for each color correspond.
In addition, the line sensor reading position is determined in real time following the input of the ITOP signal, while the data accumulation and transmission of the line sensor, the image processing for the transferred data, and the data recording process are performed for each line following the input of the ITOP signal. If the phases of the ITOP signal and the BD signal for the second color differ from those for the first color, the MTF for the reading of data for each color is varied, and the reproductivity of a fine line, for example, may be changed for each line, or the determination of edges, such as characters, may differ for each color.
The above described phenomenon will now be described in detail.
In FIG. 35, the relationship is shown between the original document, including fine lines, and the line reading positions of the line sensor for the first and the second colors.
FIG. 35 is a diagram for an enlarged portion of the original document, showing the phase relation between the BD signals for the first and the second colors and the ITOP signal, the reading start position for each color, and the data that are read. The original document is divided into squares for individual pixels beginning at the leading edge of the original, and density data for a pixel is represented in each square. Since actually, color data to be read differ for the first and the second colors, the density data provided for individual pixels are essentially different. However, for the sake of convenience, the same density data are employed for pixels of the first and the second color in this embodiment.
The data reading performed for the first color will now be described.
As shown in FIG. 35, the phase relation between the BD signal for the first color and the ITOP signal is as follows. Since the time for generation of the leading edge of the ITOP signal is the center of the generation cycle of the BD signal, and the original is read for each line in synchronization with the n-th BD signal after the ITOP signal is generated. Therefore, as shown in FIG. 35, while shifting by the xc2xd line from the square of the original, the data for the first color are read and recorded beginning with the first line. The data that are read are xe2x80x9c50xe2x80x9d, xe2x80x9c50xe2x80x9d, xe2x80x9c0xe2x80x9d and xe2x80x9c0xe2x80x9d, beginning from the first line.
The data reading for the second color will now be described.
As shown in FIG. 35, the phase relationship between the BD signal for the second color and the ITOP signal is as follows. Since the time for the generation of the leading edge of the ITOP signal is substantially the same as the timing for the generation of the BD signal, the phases of the ITOP signal and the BD signal are shifted xc2xd line away from those for the first color. Since the original document is read for each line in synchronization with the n-th BD signal following the generation of the ITOP signal, as shown in FIG. 35, while being matched with the squares of the original document, the data for the second color are read and recorded. The data that are read, beginning at the first line, are xe2x80x9c0xe2x80x9d, xe2x80x9c100xe2x80x9d, xe2x80x9c0xe2x80x9d and xe2x80x9c0xe2x80x9d.
FIG. 36 is a diagram showing the image data read for the first color and their positions, the image data read for the second color and their positions, and the relationship between the original image and an image that is reproduced based on the image data that are read.
As shown in FIG. 36, image data for the second color are read as those having the same density value as the original image because the phase of the BD signal is the same as that of the ITOP signal, as shown in FIG. 35. For the first color, however, the image data for a pixel of the original image having a density value of xe2x80x9c100xe2x80x9d are read as image data for two pixels, each of which have a density of xe2x80x9c50xe2x80x9d, because the BD signal is shifted xc2xd line away from the ITOP signal, as shown in FIG. 35.
Specifically, for the first color, a fine line in the actual original document that has a density value of xe2x80x9c100xe2x80x9d and a width equivalent to one pixel is read as a slightly thick line having a reduced density value of xe2x80x9c50xe2x80x9d and a width equivalent to two pixels, so that the resolution is slightly reduced. For the second color, the pertinent line is read as being the same as the one in the original document.
In the image forming process using the image data read for the first and the second colors, the images for the first and the second colors are superimposed to reproduce the image. Therefore, if the reproductivity differs for the images for individual colors, as in the above case wherein the image for the first color is fuzzy while the image for the second color is sharp, the fine lines of the image will be blurred, compared with those of the original image.
In addition, in the edge determination process using the image data read for the first and the second colors, the images for the first and the second colors are superimposed to reproduce the image. Therefore, if the reproductivity differs for the images for individual colors, as in the above case wherein the image for the first color is fuzzy while the image for the second color is sharp, the location that is determined for an edge will vary for each color.
This is because, since the phase relationship between the BD signal and the ITOP signal differs for the first and the second colors, the MTF of the image that is read is different for each color.
In order to reduce the shifting distance when images for individual colors are superimposed, various methods have been proposed whereby the integer number of main scan stat signals (BD signals) are obtained, while a photosensitive drum, which serves as a photosensitive member, is rotated once and a sub-scan start signal (an ITOP signal) is generated, and whereby the rotation of a drum motor that drives the photosensitive drum is synchronized with the rotation of a scanner motor that drives a polyhedron mirror for laser beam scanning.
This method will now be described in detail.
FIG. 37A is a diagram showing an example where the integer number of main scan start signals (BD signals) are obtained during one rotation of the photosensitive drum (in this example, n+0.5 signals), and where the latent images up to the third line for the second color are formed as the photosensitive drum is rotated twice. As shown in FIG. 37A, when a photosensitive drum 801 is rotated once, a sub-scan start signal (an ITOP signal) is generated by an ITOP sensor 802. The first line for the second color is shifted 0.5 line away from the first line for the first color each time the photosensitive drum 801 completes a rotation (i.e., each time an ITOP signal is generated by the ITOP sensor 802). Similarly, as the screen images for the individual colors, such as the third and the fourth colors, are superimposed, the locations of the lines for the individual screen images are shifted 0.5 line.
FIG. 37B is a diagram showing an example wherein the integer number of main scan start signals (BD signals) is obtained while the photosensitive drum is rotated once (n signals in this example), and where latent images are formed up to the third line for the second color while the photosensitive drum is rotated twice. As shown in FIG. 37B, how many times the photosensitive drum 801 rotates, the locations of the lines for the individual screen images (e.g., the position of the first line for the first color and the position of the first line for the second color) are, theoretically, matched exactly.
A specific arrangement for synchronizing the rotation of a drum motor with the rotation of a scanner motor can be an arrangement wherein a signal obtained by the frequency division of a BD signal is employed as a reference clock to control the rotation of the drum motor, or an arrangement wherein a reference clock that is used to control the rotation of the drum motor and a reference clock that is used to control the scanner motor are generated by employing a clock that is produced by a commonly used oscillator.
First, in FIG. 38 is shown the arrangement wherein a signal obtained by frequency division of a BD signal is employed as a reference clock that is used to control the rotation of the drum motor.
In FIG. 38, a photosensitive drum 3901 is rotated by a drum motor 3907 via a drive belt 3908.
Based on a reference clock CLK that is generated by an oscillator 3911, a scanner motor 3902 is controlled by a PLL circuit 3910 and rotates a polygon mirror 3903 at a constant speed.
A laser beam is emitted by a laser 3904 based on image data received from a document reader (not shown), and is reflected by the individual faces (eight faces) of the polygon mirror 3903; the reflected beam being projected, via a lens 3905, onto the photosensitive drum 3901 to form a latent image thereon. With this arrangement, as the polygon mirror 3903 is rotated once, the latent images for eight lines are formed on the photosensitive drum 3901.
A beam sensor 3906 is located in a portion other than the image forming area of the photosensitive drum 3901. The beam sensor 3906 detects the radiation of a laser beam, and generates a main scan start signal (a BD signal) each time one line is scanned by the laser beam. That is, the beam sensor 3906 produces eight BD signals while the polygon mirror 3903 is rotated once, and transmits the BD signals as a reference clock for the PLL circuit 3909 that rotates the drum motor 3907 at a constant speed. With this arrangement, the rotation of the scanner motor 3902 is synchronized with the rotation of a drum motor 3907.
Next, in FIG. 39 is shown the arrangement where a reference clock that is used to control the rotation of the drum motor and a reference clock that is used to control the scanner motor are generated by employing a clock that is produced by a commonly used oscillator.
In FIG. 39, a photosensitive drum 3001 is rotated by a drum motor 3007 via a drive belt 3008. Based on a reference clock that is generated by an oscillator 3011, the drum motor 3007 is controlled by a PLL circuit 3009 and rotates the photosensitive drum 3001 at a constant speed.
A reference clock generated by the oscillator 3011 is transmitted to a PLL circuit 3010, and in accordance with this clock, the PLL circuit controls the rotation of a scanner motor 3002, so that the polygon mirror 3003 is rotated at a constant speed.
Based on image data supplied by a document reader (not shown), a laser beam is emitted by a laser 3004 and is reflected by the individual faces of the polygon mirror 3003, which is rotated at a constant sped by the scanner motor 3002, whose rotation is controlled in the above described manner. The reflected laser is transmitted via a lens 3005 to the photosensitive drum 3001 and forms a latent image thereon.
As described above, the reference clock for the PLL circuit 3010, which controls the rotation of the scanner motor 3002, and the reference clock for the PLL circuit 3009, which controls the rotation of the drum motor 3001, are generated by using a clock that is produced by the same oscillator. Therefore, the rotation of the scanner motor 3002 is synchronized with the rotation of the drum motor 3001.
For the thus described color image forming apparatus wherein an integer number of main scan start signals (BD signals) are obtained while the sub-scan start signal (ITOP signal) is produced during one revolution of the photosensitive drum, one of the above described arrangements is employed to synchronize the rotation of the scanner motor with the rotation of the drum motor. As a result, the screen images for the individual colors can be superimposed on the photosensitive drum without their positions being shifted, and a high quality color image can be obtained.
To control the position shifting when screen images for individual colors are superimposed, there is a method, other than the above method, for which the integer number of main scan start signals (BD signals) need not be obtained while the sub-scan start signal (the ITOP signal) is generated during one revolution of the photosensitive drum. A specific arrangement for such a method will now be described in detail.
FIG. 40 is a diagram showing an example arrangement for the system of the conventional color image forming apparatus for driving a scanner motor and a drum motor.
In FIG. 40, a photosensitive drum 3101 is rotated by a drum motor 3107 via a drive belt 3108. Based on a reference clock produced by an oscillator 3114, the drum motor 3107 is controlled by a PLL circuit 3109 and rotates the photosensitive drum 3101 at a constant speed.
Each time the photosensitive drum 3101 is rotated, an ITOP sensor 3115 is shielded by a sensor flag 3116 and generates an ITOP signal. With the ITOP signal as a reference, the writing start position is determined for the first line for each screen image on the photosensitive drum 3101.
A phase matching circuit 3112 matches the phase of a reference clock generated by an oscillator 3113 with the phase of an ITOP signal produced by the ITOP sensor 3115. In accordance with the reference clock that is output by the phase matching circuit 3112 and is synchronized in phase with the ITOP signal, a scanner motor 3102 is controlled by a PLL circuit 3110 and rotates at a constant speed.
Since the phase matching circuit 3112 matches the phase of the ITOP signal with the phase of the reference clock used to control the rotation of the scanner motor 3102, each time the ITOP signal is generated, it is synchronized with the reference clock, so that the rotational phase of the scanner motor 3102 is constant. Therefore, polygon mirror 3103 is rotated by the scanner motor 3102 in synchronization with the ITOP signal, and when the laser beam from the laser 3104 is projected, via the lens 3105, onto the photosensitive drum 3101 for scanning, the laser beam scan start timing for each screen image is matched with the timing for the generation of the ITOP signal.
FIG. 41 is a diagram showing the relationship between the main scan line on the photosensitive drum and the timing for the generation of the sub-scan start signal (ITOP signal).
As shown in FIG. 41, each time the photosensitive drum 3601 is rotated, the main scanning for n+0.5 lines is performed, and the ITOP sensor 3602 generates an ITOP signal at a predetermined timing. With this arrangement, since the main scanning is performed for n+0.5 lines during one revolution of the photosensitive drum, the first line during the second rotation (second color) of the photosensitive drum will be shifted 0.5 line away from the first line during the first rotation (first color). With the arrangement shown in FIG. 40, however, the phase matching circuit 3112 matches the phase of the ITOP signal with the phase of a reference clock that is used to control the rotational phase of the scanner motor 3102, and the rotational phase of the scanner motor 3102 is controlled in accordance with the reference clock. Therefore, as shown in FIG. 41, the start timing for scanning the first line can be matched each time the photosensitive drum is rotated. Further, no matter how many times the photosensitive drum rotates, the main scan lines of the screen images are not shifted on the photosensitive drum, so that the screen images for the individual colors can be superimposed exactly.
The rotations of the photosensitive drum and the polygon mirror are slightly fluctuated by, for example, a change in loads on the drum motor and the scanner motor, or a backlash produced by a mechanical drive transmission system, such as a set of gears. When the phase relationship between the main scan start signal (a BD signal) and the sub-scan start signal (an ITOP signal) fluctuates due to a change in the rotation of the photosensitive drum or the polygon mirror, as was mentioned previously, the MTF is varied for an image for each color that is read from the original document. Further, to form the screen images for individual colors on the photosensitive drum, the fluctuation appears as position shifting among the individual screen images. As a result, the quality of an image that is formed by superimposing the screen images deteriorates, and an erroneous edge determination is acquired.
The change in the phase relationship between the main scan start signal (a BD signal) and the sub-scan start signal (an ITOP signal) can be suppressed within a one line interval or shorter by minimizing the change in the load placed on the drum motor or the scanner motor, or by improving the accuracy of a mechanical drive transmission system, such as set of gears. Assume that, as shown in FIG. 42, in the process of forming a screen image for each color on the photosensitive drum, the time for the generation of an ITOP signal falls before or after the time for the generation of the first BD signal. In this case, the ITOP signal generation timing is shifted from the generation timing for the first BD signal by a period of time that is equal to or shorter than the one line time period; however, the position shifting that is equivalent to one line occurs in a color image obtained by superimposing the screen images for individual colors.
As described above, in FIG. 42, during the first rotation of the photosensitive drum, an ITOP signal is generated immediately before the scanning timing (i.e., the BD signal generation timing) for the first line of the screen image for the first color that is formed on the photosensitive drum, and for the second rotation of the drum, an ITOP signal is generated immediately after the scanning timing for the first line of the screen image for the second color that is formed on the photosensitive drum.
As shown in FIG. 42, a photosensitive drum 1201 is rotated, and an ITOP sensor (not shown) is shielded by a sensor flag 1202, so that an ITOP signal is produced by the ITOP sensor. At the first rotation of the photosensitive drum, the ITOP signal is generated immediately before a BD signal (1) that indicates the timing for the start of a scan for the first line in the screen image of the first color. Therefore, the first line is scanned in synchronization with the first BD signal (BD signal (1)) that is generated after the ITOP signal is generated, and then the second line is scanned in synchronization with BD signal (2). In this manner, the photosensitive drum is scanned by a laser beam, and the screen image for the first color is formed. At the second rotation of the photosensitive drum, an ITOP signal is generated immediately after BD signal (1), which indicates the scan start timing for the first line of the screen image for the second color. Therefore, the first line is scanned in synchronization with the first BD signal (BD signal (2), in this case) that is generated after the ITOP signal is generated, and then the second line is scanned in synchronization with BD signal (3). Since the photosensitive drum is scanned in this manner by a laser beam, the original reading position and the writing position relative to the photosensitive drum for the second rotation of the drum are shifted one line away from those for the first rotation.
To resolve the above shortcomings, it is an object of the present invention to provide an image forming apparatus that can maintain a constant original reading start position and a constant image forming start position for each color element, and that can obtain a high quality color image with less variance in the reproductivity of individual color element images; and a control method therefor.
To achieve the above object, according to the present invention, an image forming apparatus comprises:
scanning means for scanning a color image for each color element;
reading means for reading image data for each line through the scanning performed by the scanning means;
a rotary polygon mirror for deflecting, in a main scan direction, a modulated light beam that is based on the image data for each line that are read by the reading means, so that an image holding member that is rotated in a sub-scan direction is scanned by the deflected light beam and a latent image is formed thereon;
first generation means for determining whether the light beam deflected by the rotary polygon mirror has scanned a predetermined location, and for generating a main scan start reference signal;
second generation means for generating a sub-scan start reference signal in synchronization with the rotation of the image holding member;
first control means for controlling the rotary polygon mirror and/or the rotary image holding member, so as to maintain a phase difference between the sub-scan start reference signal, produced by the second generation means, and the main scan start reference signal, produced by the first generation means;
second control means for, in synchronization with the sub-scan start reference signal generated by the second generation means, starting to drive the scanning means;
third control means for, in synchronization with the main start reference signal generated by the first generation means, starting the reading of an original document by the reading means; and
fourth control means for, in synchronization with the main start reference signal generated by the first generation means, starting to use the light beam to form a latent image.
It is another object of the present invention to provide a high quality image forming apparatus wherein, even when the phase relationship of a main scan start signal and a sub-scan start signal is slightly changed due to a change in the rotation of an image holding member or a rotary polygon mirror, the occurrence of a shift in the phase relationship of these scan signals that is equal to or greater than the actual change can be prevented, and wherein the MTF for each image that is read from the original document can be prevented from varying, and image deterioration due to the shifting of superimposed images or an erroneous edge determination does not occur.
To achieve the above object, an image forming apparatus, which forms a composite image by superimposing a plurality of screen images, comprises:
original reading means for employing a line sensor to scan an original document a plurality of times and for outputting image data acquired during each scan;
light beam generation means for generating a light beam based on the image data output by the original reading means;
image holding member driving means for rotating an image holding member;
a rotary polygon mirror for, while rotating, reflecting the light beam emitted by the light beam generation means, so as to scan the image holding member that is rotated by the image holding member driving means;
main scan start signal generation means for generating a main scan start signal that indicates a rotation timing for the rotary polygon mirror; and
sub-scan start signal generation means for generating a sub-scan start signal that indicates a rotation timing for the image holding member,
wherein a timing at which the main scan start signal is generated by the main scan start signal generation means, and/or at which the sub-scan start signal is generated by the sub-scan start signal generation means, is controlled, so that a phase relationship of the main scan start signal and the sub-scan start signal, which is detected at a first timing, is also maintained for a second timing that differs from the first timing, and
wherein the scanning, using the line sensor, of the original document by the original reading means, and the scanning, using the rotary polygon mirror, of the image holding member with the light beam are controlled in accordance with the main scan start signal, generated by the main scan start signal generation means, and the sub-scan start signal, generated by the sub-scan start signal generation means.
The other objects and features of the present invention will become apparent during the course of the following description, given while referring to the accompanying drawings, and the description of the claims.